7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Pulsative non-thermal emissions between ion cyclotron and electron cyclotron
frequencies during external heating in magnetically confined experimental
plasmasH. Igami1, M. Toida1, A. Fukuyama2, S. Inagaki3, K. Nagasaki3, T. Tokuzawa1, H. Yamaguchi1, S.
Murakami2, R. Seki1, H. Nuga1, H. Kasahara1, T. Seki1, K. Nagaoka1, S. Kobayashi3, M. Goto1, Y.
Katoh4, and N. Kenmochi11National Institute for Fusion Science, 2Graduate School of Engineering, Kyoto University,
3
Institute of Advanced Energy, Kyoto University,
4
Graduate School of Science, Tohoku University
e-mail (speaker): igami.hiroe@nifs.ac.jp

Pulsative intensity increase and decrease of the nonthermal emissions are observed in the harmonicfrequency range of the ion cyclotron, lower hybrid,and electron cyclotron waves at helical devices.
In the large helical device (LHD), at the start-up
phase of the plasma discharge initiated by tangential
neutral beam and electron cyclotron wave, pulsative
zebra-like pattern emissions are observed as shown inFig. 1. The peak frequencies correspond to f = n fLH + mfIC, where fLH is the lower hybrid and fIC is the ion
cyclotron frequency. It was previously suggested thatwhen the lower hybrid frequency is close to theinteger multiple of the ion cyclotron frequency, wavesof harmonic ion cyclotron frequencies are excited withthe existence of the high energy ions that have low
pitch angle [1] and can be provided by tangential
neutral beam injection. Here, the existence of waves ofharmonic frequency range of the lower hybrid wave isreported. We have found that there are two speciallyseparated double resonances “DBR1” and “DBR2”
where the ion cyclotron resonance and the lower
hybrid resonance overlap. At each double resonance,𝑓!" is approximately a multiple of 𝑓#$ . We have also
found that 6fIC at DBR1 corresponds to fLH at DBR2.

Fig.1 : Time change of the frequency spectrum
and signal intensity detected by RF probe at theinitiation phase of the plasma discharge in LHD

The impact of the waves that propagate between DBR1and DBR2 on the intensity of the waves excited ateach double resonance via nonlinear wave-wave
coupling will be investigated.
After the appearance of the emissions describedabove, pulsative intense non-thermal emissions areobserved with chirping-up/ down of the frequency inthe second harmonic electron cyclotron frequency
range. In Heliotron J, similar emissions are also
observed when the plasma is initiated only by 2nd
electron cyclotron wave injected obliquely to theexternal magnetic field as shown in Fig. 2. The intenseemission is observed under the 2nd ECRH frequency.
This time change of frequency spectrum looks like thefalling tone of the whistler-mode wave in themagnetosphere although the condition of 𝜔%& ⁄Ω'& >1 is not satisfied in the experimental condition.
Interaction between high energy electrons andexisting waves will be investigated.
References[1] M. Toida, H. Igami, et al, Plasma Fusion Res. 14,3401112 (2019)

Fig.2 : Time change of the frequency spectrumof 2nd electron cyclotron wave range at theinitiation phase of the plasma discharge inHeiotron J